This invention relates to laser beam processing of materials, and more particularly, to a method and apparatus for laser beam drift compensation in a materials processing machine.
Manufacturing and materials processing machines that employ lasers for their source of processing energy are subject to variations in the laser because of temperature, vibration, power-up and shutdown of the laser source, and random drifting of the laser pointing direction. Over relatively short periods of time, the precise direction of laser beam wanders slightly. This laser beam characteristic appears to be random and cannot be predicted.
Many laser processing machines use acousto-optic deflectors to deflect or scan the laser beam. Acousto-optic deflectors are sensitive to their immediate past use. It appears that heating of the piezoelectric deflector crystals that make up the heart of the acousto-optic scanner depends on the deflection required from each of the x or y deflectors. That is, if deflection is required at the upper-right of the field of scan, the x and the y deflector crystals will heat up a certain amount. If deflection is required in another part of the scan field, the crystals will heat up to a different degree. As a pattern is written by scanning the laser beam, the crystals heat up in varying amounts according to the particular pattern. The variations in heating cause beam shape distortion, pin cushion distortion, and pointing direction errors depending on the difference in the temperature of the crystals at the time of the last alignment of the laser beam and the temperature at the time of writing. These deflection distortions are dependent on the pattern written.
The deflection distortions discussed above are unimportant in many laser processing applications, but can easily amount to several micrometers of error on the working surface of the workpiece to be processed by the laser. Employing a laser processing machine to write patterns on an integrated circuit surface, which is used to make gate arrays, may cause deflection distortions that are intolerable.
It is possible, but very expensive, to design the mountings of passive optical components to eliminate virtually all effects of temperature variations. The expense and difficulty of insuring temperature independence of the optical components is cost prohibitive in all but the most exacting laser processing machines.
Accordingly, there is a need for an inexpensive method and apparatus to compensate laser beam drift.